Investigators in the Divisions of Clinical Pharmacology and Cardiology at Duke University Medical Center have used the Rankin Clinical Research Unit to study patients with atrial fibrillation and other common disorders of the heart rhythm. Atrial fibrillation is the most common disorder of the heart rhythm requiring treatment with antiarrhythmic drugs, and it accounts for over one-third of all U.S. hospitalizations for arrhythmias. Patients with atrial fibrillation have their usually normal heart rhythm replaced by periods of rapid, irregular heart beating that may cause a sense of pounding in the chest, shortness of breath, chest pain, dizzyness, or loss of conciousness. These periods may last for minutes or hours, or they may last indefinitely unless acute treatment is given by a physician. Studies conducted on the Rankin Clinical Research Unit have concentrated on elucidating the mechanism, clinical course, and optimal treatment of atrial fibrillation and related disorders of the heart rhythm. Recent studies have focused on measuring the frequency of spontaneous occurrence of atrial fibrillation and paroxysmal supraventricular tachycardia. Among 150 patients studied who had their antiarrhythmic drug therapy withdrawn, one-half had a spontaneous, symptomatic recurrence in about 3 weeks; in fact, about 1 in 5 had a symptomatic recurrence of their arrhythmia in less than 1 day. The average heart rate during recurrences was 132 beats/min for patients with atrial fibrillation and 198 beats/min for patients with paroxysmal supraventricular tachycardia. Among the patients with atrial fibrillation, about 1 in 9 had a second arrhythmia, atrial flutter, recorded along with fibrillation. Asymptomatic arrhythmias were common. The rate of asymptomatic arrhythmia events was 12 fold higher than the rate of symptomatic events.